The present invention relates to constrained hardening of bar-shaped workpieces for preventing warping. In particular, this invention is directed to a hardening method and apparatus suitable for odd-shaped workpieces such as steering rack bars for automobiles which have a tendency to warp during hardening.
Hardening of bar shaped machinery parts is very popular. In order to harden axially symmetrical parts such as a body of simple revolution, rotation around the axis during quenching for equalization of cooling is adopted for prevention of warping. Moreover prevention of warping by exerting constraining force is also adopted. For example, a round bar is rotated among three rolls extending therealong during quenching as shown in Japanese laid-open patent No. Sho 54-67504.
However, bar workpieces having a deformed (odd) shape such as a bar workpiece having a rack gear along its side tend to bend by hardening because the cooling rate is not uniform about their axes. Moreover, these workpieces mentioned above cannot be rotated during quenching, so that press quenching is usually used for prevention of warping during hardening. In the press quenching, all or a part of a workpiece heated to an appointed hardening temperature is held in a set of dies having determined cavities. Then the workpiece is cooled down under pressure by a method such as immersing into a cooling liquid tank.
Recently there has been a trend of cutting weight of automobiles by hollowing such aforementioned bar parts with a rack gear along its side to form pipe parts. However, the pipe parts are very liable to create bending during the hardening process compared with solid bars. Moreover, the pipes could be collapsed by a pressing force in the aforementioned press quenching. If the pressing force is limited for preventing collapse, warping might not be suppressed and strain might arise when being pressed in the dies after being cooled down. Moreover, even if the pressing force is considerably increased and the workpiece is made straight in the dies, warping might arise by existence of residual stress as the workpiece is taken out from the dies. Such residual stress is usually controlled by regulating a cooling rate by partly changing a cooling liquid amount blown against a workpiece depending on the workpiece shape. However, occurrence of warping is not so simple as it changes depending on steel constituents in relation to transformation temperature, because strain during the hardening process is a sum of thermal strain and transformation strain.